Have a look at this thread as well should give you some more info
http://www.pprune.org/forums/showthread.php?t=212347
I'm looking to maybe go down the same route when i get my PPL .
Budget on 290 / 300 an hr in the mk3 JP and 400 ish in the mk 5
Say you need 10 - 15 hrs to complete the course your looking on £4000 for the converstion .
If you always fly with a friend and split the cost its £150 an hour after you pass the course.
Most flights i've been told last about 45 min so your probably looking at £125 an hr each .
Not bad when you can blast around the country side at 400mph.